1. Field of the Invention
The present invention relates generally to a flash of a photographic device, and more particularly to a charging device for the flash.
2. Description of the Prior Art
A flash is a component disposed in a photographic device, such as a digital camera, a digital video camera, and the like, to produce extremely brightlight in a very short duration in indoor or other low light conditions. The earliest such camera flashes are consisted of a wad of magnesium powder that is ignited by hand. Later, magnesium filaments are contained in flash bulbs and electrically ignited by the camera shutter. Such flash bulbs could only be used once, and are too hot to handle immediately after use. Today's camera flash is comprised of a sealed tube which is filled with a mixture of gases, primarily Xenon, and electrodes to carry electrical current to the gas mixture. Additionally, a high voltage power source is necessary to energize the gas mixture. This high voltage is usually stored on a capacitor so as to allow very speedy delivery of very high current when the camera flash is triggered. Once the camera flash flashes, it consumes power. Therefore, to keep the camera flash working, there is a need of a charging device (or a charging circuit) for providing the camera flash with electricity.
The charging device often has a flash capacitor. Currently, there are two common methods for charging the flash capacitor. One method is to adopt self-oscillation. A conventional charging circuit working in the self-oscillation mode is disclosed in U.S. Pat. No. 5,966,552. This conventional charging circuit comprises a booster section for boosting a voltage up to a set voltage for the flash capacitor to use, and a charging-discharging section for charging the flash capacitor. However, the charging circuit comprises a number of capacitors, resistors, transistors and diodes to form a switch circuit, a self-oscillation circuit, a filtering circuit, a voltage transforming circuit, a rectifying circuit, a discharging circuit, and so on. Obviously, the circuit integration is very complicated. Especially, the self-oscillation circuit and the filtering circuit occupy too much space of the circuit board, which goes against the simplification and miniaturization trend of the circuit board. Furthermore, the charging efficiency of this conventional charging circuit is not high enough to meet the high efficiency requirements of today's electronic photographic devices.
The other method for charging a flash capacitor is to use a specialized integrate circuit (IC) chip. Such a method and the associated charging device are disclosed in U.S. Pat. No. 6,714,735. The charging device as disclosed comprises a flash capacitor, a voltage transforming circuit electrically connected with the flash capacitor for providing an induced current to charge the flash capacitor, a pulse width modulation (PWM) control circuit electrically connected with the voltage transforming circuit for providing a pulse voltage to the voltage transforming circuit that is driven to provide the induced current. However, the PWM control circuit is integrated onto a specialized IC chip that is specially used to provide PWM voltage signals. The PWM control circuit is electrically connected with a digital signal processor (DSP) in a photographic device. As is known, an IC chip is much more expensive than other basic components in a circuit. Therefore, using the IC chip inevitably increases the cost of the charging device. Furthermore, the PWM control circuit is under control of an additional charging control circuit. The charging control circuit not only occupies a certain space of the circuit board but also increases the cost of the charging device, thus increasing the volume and the total cost of the photographic device.
Hence, an improved charging device for a camera flash is required to overcome the above-mentioned disadvantages of the prior art.